Storm Activity Part of a Cycle

Charley. Frances. Now Ivan.

The seemingly nonstop pummeling of Florida during the last month has been the result of a highly active hurricane season mixed with two unusually stubborn high-pressure ridges funneling the storms in a track directly over the state.

It is also a taste of things to come.

“It’s an overall increase in hurricane activity and bad luck for Florida,” said Chris Landsea, a researcher at the National Oceanic and Atmospheric Administration’s Hurricane Research Division in Miami. “It’s not really a big surprise.”

In a normal year, high-pressure ridges and low-pressure troughs fluctuate in their location. Hurricanes, which gravitate toward low-pressure areas, typically hit land along a wide swath stretching from Mexico to the Middle Atlantic States.

This year, a low-pressure trough has hovered over Florida for weeks. High-pressure ridges to either side of the peninsula keep herding the storms right toward the state.

“The ridges and troughs are set up so they’re aimed right at Florida this year,” Landsea said.

Florida has been hit by three hurricanes in a season three times in the last 100 years: in 1926, 1950 and 1964, according to NOAA.

The high number of storms this year was expected. Hurricane activity rises and falls in cycles that last 20 to 40 years, and a cycle of high hurricane activity has persisted since 1995, said Jim Laver, director of NOAA’s Climate Prediction Center in Camp Springs, Md.

William Gray, a professor of atmospheric science at Colorado State University, said he was surprised that Florida hadn’t experienced even more serious storms during this hurricane cycle.

“Florida has certainly had a lot of problems in the last 3 1/2 weeks. But Florida has been extremely lucky with regards to these major storms,” Gray said.

The fluctuations in hurricane activity are caused by cyclic changes in oceanic and atmospheric patterns that are not fully understood. But in periods of high activity, ocean water is warmer. This heat fuels the creation of hurricanes.

In high-activity periods, the storms are less likely to fall apart, because there is less wind shear to destroy them, Laver said.

Laver’s center predicted in March that this season had a 90% chance of being normal or above normal. He expects to see 10 to 20 more years of heavy hurricane activity.

“We have every reason to believe seasons to come will be active,” he said, adding that strong El Nino events could moderate hurricane seasons because they tend to increase the wind shear that rips hurricanes apart.

Some previous reports suggested that global warming could lead to more and stronger hurricanes and even to “hypercanes,” with winds the speed of sound, or about 750 mph.

That is just not the case, hurricane researchers say.

“None of this has anything to do with global warming,” Laver said.

“These are natural cycles that we see.”

Climate change is predicted to increase the temperature of ocean waters, but it also is expected to increase air temperatures.

That means the contrast between air and water temperatures, which helps build hurricanes, probably would not change much.

One climate model suggested that even at levels of warming caused by a doubling of the greenhouse gas carbon dioxide, hurricane intensity would increase by just 5%, Landsea said.

According to most models, the number of storms and the location of their occurrence are not expected to change because of global warming, he said.

The hurricane season lasts until late November, so Florida may have to brace for more storms.

But the post-Ivan outlook is quiet.

“We’re not tracking anything else,” Landsea said.

“I’m hoping we get a break. The state of Florida could really use it.”

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Times staff writer John-Thor Dahlburg in Florida contributed to this report.